Well fracturing method employing a liquified gas and propping agents entrained in a fluid

ABSTRACT

The formations surrounding a well bore are subjected to hydraulic fracturing. A liquified gas and a fluid containing entrained propping agents are injected into the formations. The liquified gas returns to its gaseous state and is therefore easily removed from the formation.

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Bullen 1 51 May 23, 1972 4 WELL FRACTURING METliOl) 3,396,107 8/196811111 ..l66/308 x A G s AND 2,896,717 7/ 1959 Howard ..166/28l 3,368,6272/1968 11 1 1 a1. ..166/3o8 x PROPPING AGENTS ENTRAINED IN A 3,108,63610/1963 P328311 166/308 FLUID 3,170,517 2/1965 Graham et a]. 166/308 x[721 lnvenm" Bulk", Calgary Albem' 2,596,844 5/1952 Clark 166/308 uxCanada v [73] Assignee: Dresser Industries, Inc., Dallas, Tex. PrimaryExaminer'"stephen Novosad Attorney-Robert W. Mayer, Thomas P. Hubbard,Jr., Danlel [22] Filed: Aug. 17, 1970 Rubin, Raymond T. Majesko, Roy L.Van Winkle, William E.

[ 1 pp 64,271 Johnson, Jr. and Eddie E. Scott ABS (ACT [52] US. Cl.166/283, [66/308, 166/75 The formations Surrounding a we bore aresubjected to [51 1 Int. 6 hydraulic fracturing A gas and a containingen- Field of Search 281, trained propping agents are injected into theformations, The 166/283 liquified gas returns to its gaseous state andis therefore easily removed from the formation.

[56] References Cited 14 Claims, 6 Drawing figures UNITED STATES PATENTS3,136,361 6/1964 Marx ..l66/308 ALCOHOL l3 PROPPING BLENDER 2 AGENT l6PUMP PUMP WELL PATENTEUMM 23 m2 8,664, 12 2 SHEET 3 [IF 3 METHANOL GELBREAK 5o 1 I I GEL VISCOSITY 480p C zo 2 METHANOL VISCOSITY 0.6cp L l l-l JLL .L 4 .E-. L E- E 0 IO so so e0 :oo no I20 I30 I40 :50 TIME INMINUTES & J

FIG. 4

ALCOHOL l3 l4 2 8 ZQ w BLENDER 00 PUMP f PUMP L IS f 25 PIC 15 T wELL 6INVENTOR 26 RONALD S. BULLEN ATTORNEY WELL FRACTURING METHOD EMPLOYING ALIQUIFIED GAS AND PROPPING AGENTS ENTRAINED IN A FLUID BACKGROUND OF THEINVENTION This invention relates to the art of hydraulically fracturingsubterranean earth formations surrounding oil wells, gas wells, andsimilar bore holes. In particular, this invention relates to hydraulicfracturing utilizing a liquified gas and a fluid containing entrainedpropping agents.

Hydraulic fracturing has been widely used for stimulating the productionof crude oil and natural gas from wells completed in reservoirs of lowpermeability. Methods employed normally require the injection of afracturing fluid containing suspended propping agents into a well at arate sufficient to open a fracture in the exposed formation. Continuedpumping of fluid into the well at a high rate extends the fracture andleads to the build-up of a bed of propping agent particles between thefracture walls. These particles prevent complete closure of the fractureas the fluid subsequently leaks off into the adjacent formations andresults in a permeable channel extending from the well bore into theformations. The conductivity of this channel depends upon the fracturedimensions, the size of the propping agent particles, the particlespacing, and the confining pressures.

The fluids used in hydraulic fracturing operations must have filter lossvalues sufficiently low to permit build-up and maintenance of therequired pressures at reasonable injection rates. This normally requiresthat such fluids either have adequate viscosities or contain filter-losscontrol agents which will plug the pores in the formation. The use offracturing fluids having relatively low viscosities in conjunction withadditives which provide the low filter-loss values needed avoidsexcessively high friction losses in the tubing and casing. The well headpressures and hydraulic horsepower required to overcome such frictionlosses may otherwise be prohibitive.

Fracturing of low permeability reservoirs has always presented theproblem of fluid compatibility with the formation core and formationfluids, particularly in gas wells. For example, many formations containclays which swell when contacted by aqueous fluids causing restrictedpermeability,

and it is not uncommon to see reduced flow through gas well cores testedwith various oils. 1

Another problem encountered in fracturing operations is the difficultyof total recovery of the fracturing fluid. Fluids left in the reservoirrock as immobile residual fluid impede the flow of reservoir gas orfluids, to an extent that the benefit of fracturing is decreased oreliminated. The removal of the fracturing fluid may require theexpenditure of a large amount of energy and time, consequently thereduction or elimination of the problem is highly desirable.

DESCRIPTION OF THE PRIOR ART In attempting to overcome the filter-lossproblem, gelled fluids prepared with water, diesel and similar lowviscosity liquids have been useful. Such fluids have apparentviscosities high enough to support the propping agent particles withoutsettling and yet low enough to give acceptable friction losses. Thegelling agents also promote laminar flow under conditions whereturbulent flow would otherwise take place and hence in some cases, thelosses may be lower than those obtained with low viscosity-base fluidscontaining no additives. Certain water-soluble poly-acrylamides, oilsoluble poly-isobutylene and other polymers which have little effect onviscosity when used in low concentration can be added to the ungelledfluid to achieve similar benefits.

In attempting to overcome the problem of fluid compatibility whenaqueous fracturing fluids are used, chemical additives have been usedsuch as salt or chemicals for pH control. Salts such as NaCl, KCl, orCaCl, have been widely used for fracturing water sensitive formations.Where hydrocarbons are used, light products such as gelled condensatehave seen a wide degree of success, but are restricted in use due to theinherent hazards of pumping volative fluids.

Low density gases such as CO, or N; have been used in attempting toovercome the problem of removing the fracturing liquid. The low densitygasses are added at a calculated ratio which promotes fluid flowsubsequent to the fracturing. This .back flow of load fluids is usuallydue to reservoir pressure alone, without mechanical aid from surface,because of the reduction of hydrostatic head caused by gasifying thefluid.

SUMMARY OF THE INVENTION The present invention provides a method of wellstimulation with little or no reservoir contamination and a highpercentage of load fluid recovery. A liquified gas and a fluidcontaining entrained propping agents are injected into the formations.Since the two aforementioned fluid phases are completely miscible theymay be either blended prior to well entry, or injected separately andblended in the well. The fluids are injected until a fracture ofsuflicient width to produce a highly conductive .channel has beenformed. Particles of the propping agent, suspended in the mixture, arecarried into the fracture. The injected fluid is then permitted to leakoff into the formation until the fracture has closed sufficiently tohold the particles in place.

Inone embodiment of the invention, liquid carbon dioxide (CO and a highconcentration of propping agents in a stream of gelled alcohol aresimultaneously injected into the well bore. When the liquid carbondioxide reaches the formations, it gasifies, leaving only a low fluidresidual of alcohol to recover. This alcohol, in turn, is soluble inreservoir gas (methane) and is essentially returned as a vapor. Thepropping agents are added to a separate side stream of alcohol atatmospheric pressure and subsequently blended with the liquid carbondioxide for injection into the well. A suitable alternate to alcohol isa light oil, condensate, or reformate (aromatic refinery by-product)gelled with additives such as aluminum stearate and time-dependantbreakers.

It is therefore an object of the present invention to provide a methodof fracturing the formations surrounding a well bore wherein proppingagents contained in a suitable fluid are added to a liquified gas andinjected into the formations.

It is a still further object of the present invention to provide afracturing method wherein propping agents are added to a suitable fluidat atmospheric pressure and the fluid containing the propping agents issubsequently mixed with a liquified gas and injected into the formationssurrounding a well bore.

It is a still further object of the present invention to provide a wellfracturing method that prevents any fluid of questionable compatibilityfrom contacting either the formation or reservoir fluids.

It is a still further object of the present invention to provide a wellfracturing method that allows extension of the shut-in period of thewell to an indefinite period of time for fracture healing, also allowingflow-back and evaluation at the operators convenience.

It is astill further object of the present invention to provide a wellfracturing method that includes a combination of alcohol, surface activeagents and liquified carbon dioxide to be injected into the formationsurrounding a well bore.

The above and other objects and advantages will become apparent from aconsideration of the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts a pressure enthalpychart for CO in the re- I gion of interest of oil well servicing.

FIG. 2 shows the viscosity of CO FIG. 3 shows the thermodynamicproperties of saturated carbon dioxide.

FIG. 4 shows the rate of reaction of gel breaker on the gelled alcohol.

FIG. 5 is a schematic representation of a system used in this invention.

FIG. 6 shows a fracturing manifold that may be used to inject the fluidsinto the well bore.

In the preferred embodiment of the present invention liquid carbondioxide (CO is the primary fracturing fluid in a well fracturing method.Simultaneous injection of gelled alcohol (methanol) is used to carry thepropping agent.

Referring now to FIG. 1, a pressure enthalpy chart for carbon dioxide inthe region of interest in oil well servicing is shown. The probable pathfollowed during a fracturing job is depicted by a dash line. Liquidcarbon dioxide is pumped from a delivery transport at approximately 300psi and F. It is pumped to an elevated pressure where it is comingledwith gelled alcohol and propping agents. The temperature of the combinedfluids rises due to mixing with warm fluid and as the mixed stream goesdown the well, it picks up additional heat from the borehole andadditional pressure due to hydrostatic head. At the perforation,pressure is at its peak and it declines after formation fracturing asfluid enters the reservoir. As the critical temperature of the carbondioxide (87.8 F.) is exceeded, it changes to the vapor stage. Whenpressure is relieved at the well head after completion of the treatment,the gaseous carbon dioxide expands along a path similar to that shown,until it emerges as a gas at the well head at atmospheric pressure.

Referring now to FIG. 2, the viscosity of carbon dioxide determined bythe method of Uyehara and Watson" was calculated over the range of 0 to300 F. for pressures from 100 to 30,000 psi. This information may beused in calculating the friction pressure drop which may be encounteredwhen pumping pure liquid carbon dioxide into a well. The density ofcarbon dioxide is calculated from the equation PV 0.243ZTM where Pequals pressure in psia; V equals volume in cubic feet; Z equalscompressibility factor; T equals temperature in degrees Rankin; M equalsweight in pounds. This equation was solved for temperatures from 0 to200 F. and pressures from 100 psi to 10,000 psi.

Knowing viscosity and density, friction pressures of liquid carbondioxide may then be calculated using Crittendon's correlation forpressure drop in oilfield production pipe:

where P/L pressure drop per l ,000 feet p density, gm/cc u viscosity, cp

Q injection rate, BPM

D pipe diameter, inches An example of friction drop for carbon dioxideat high pressures was calculated at BPM for 3.548 in. I.D. tubing at6,000 psi indicating that the pressure drop for carbon dioxide underthese conditions is only 43 percent less than that of water with 1 cpviscosity. The addition of the gelled alcohol and sand slurry to thecarbon dioxide injected into a well does not appear to change the pipefriction appreciably from that calculated, although variations inperforation friction are proportionate to the increased density of theslurry according to the equation:

P perforation friction, psi

Q injection rate, BPM

p density, gm/cc n diameter of perforation, inches D diameter ofperforations in inches Orifice coefficient assumed 0.8

The thermodynamic properties of saturated carbon dioxide are shown inthe Table of FIG. 3. As may be seen from this Table, the latent heat ofvaporization is a function of temperature, ranging from 120.1 BTU/lb. at0 F. (transport conditions) to 0.0 BTU/lb. at 87.8 F., when the CO isentirely gaseous.

In a fracture treatment using carbon dioxide as a base fluid, the totalheat absorption from the tubing, casing and formation for a stimulationincorporating 10,000 gallons of liquid carbon dioxide would therefore be1.0 X 10 BTU. This is usually well within the tolerable range forcooling effect corresponding to an average temperature drop throughoutthe system of only 20 F. to 30 F. which is quickly replaced by downholeheat transfer to equilibrium.

Under reservoir temperature and pressure conditions, the specific volumeof carbon dioxide increases from that at the surface. This volumeexpansion increases the velocity of the fracturing fluid in theformation for improved fracture width and penetration. The volumeoccupied by 1,000 SCF of carbon dioxide prior to injection is 1.78 ft.(0 F 300 psi). This volume expands under reservoir conditions, with thegreatest effect at lower pressures.

The volume of gaseous carbon dioxide in the well reservoir at anytemperature may also be calculated according to the formula:

m X tn/p where V is the final volume,

V is the initial volume at standard conditions,

f is the compressibility factor at final conditions,

p is the final pressures in atmospheres.

The gelled alcohol used to carry the propping agent may be methanol oranother alcohol with similar properties. Methanol is used as a proppantcarrying agent in view of its compatibility with most gas and oilreservoirs, its low freezing point, and potential chemical benefits tothe stimulation.

As shown in FIG. 4 the rate of reaction of gel breaker on the gelledalcohol is rapid, but allows adequate time for the displacement of theproppant into the formation at a high viscosity blend. From initialviscosity of 50 to 60 cp the gel breaks back to 2 cp as a finalviscosity. By comparison, straight methanol has a viscosity of 0.6 cp.It is preferred that the alcohol be gelled to a viscosity of 20 cp orhigher in order to be sufficient to carry the high concentration ofpropping agent through the pumping equipment and into the formation. Ina low fluid residual treatment, the alcohol occupies as little as 16percent of the total fluid volume. This alcohol is distributed over atotal fracture area of many thousands of square feet, and in actualfield use it is seldom recovered as a liquid.

Total recovery of the methanol without residual fluid saturation isrealized by vaporization during subsequent production of the well. Thesaturation of alcohol in methane varies with temperature and pressure,but is generally over 250 lbs. per million cubic feet of gas underreservoir conditions.

Referring now to FIG. 5 one embodiment of a system of the presentinvention is shown in schematic form. An alcohol storage tank 11 isconnected to a blender 12. The blender 12 may be of the typeconventionally used in oil field fracturing operations and wouldnormally include paddles, a ribbon mixer or jets for mixing andsuspending propping agents in the gelled alcohol. The alcohol is gelledin this blender just prior to the addition of the propping agents. It isgenerally preferred to operate the blender 12 at a high speed to preventbuildup and slugging of the propping agent particles. A return line 13from the blender to the alcohol storage tank 1] permits circulation topromote initial mixing of the fluid before the propping agent is added.A suitable propping agent from container 14 is added to blender 12.Discharge line 15 extends from blender 12 to high pressure fracturingpump or pumps 16. These pumps are normally positive displacement,Triplex pumps, truck mounted and specially equipped for pumping abrasiveslurries at high rates and pressures.

Liquid carbon dioxide from tank or tanks 17 is injected into the well 18by means of a pump or pumps 19. Unit 19 may be a pumping unit similar tothat described in connection with pumping unit 16.

The pumps 16 and 19, blender 12, tanks 11 and 17 and other equipment arenormally located some distance from the well 18 to minimize the dangerin case of fire or blowout. Valves are provided throughout the system topermit control of the fluids and the disconnection of individual unitsof equipment as necessary.

Referring now to FIG. 6 another embodiment of the present invention isshown. A fracturing manifold particularly suited for the presentinvention is indicated generally at 20. Thegelled alcohol with proppantenters the manifold 20 at the inlet 21. It receives the gel breakingmixture which enters at 22 and the mixture then passes down the tubing23. The liquid carbon dioxide enters at 24 and passes down the annulusbetween tubing 23 and tubing 25. As the gelled alcohol including theadditives exits from tubing 23 it is completely mixed with the carbondioxide prior to entry into the formation 26.

in a typical treatment, alcohol with between 5 to 8 lbs. of proppant pergallon, depending on the well conditions, is pumped into a manifold atthe rate of 7 barrels per minute. The carbon dioxide is pumped at 14barrels per minute into the manifold and a diluted sand/liquid ratio ofapproximately 2 lb./gal. is injected into the well. Additional additivessuch as surfactants and fluid loss additives may be added to the alcoholat the blender during the treatment.

When injecting the alcohol and carbon dioxide separately into the tubingand casing, a controlled screen-out may be effected at the conclusion ofthe fracture treatment. Simultaneous with the flush, the annulus carbondioxide rate is reduced to increase the bottom hole sand concentration.After the sand is displaced into the formation, the well is shut in forany length of time desired prior to the flowing back to evaluatetreatment.

It is generally recognized that the stimulation benefits resulting fromthe present invention are two-fold:

1. highly permeable channels are developed which have the effect ofallowing increased flow into the well bore; and

2. the well bore area itself is cleaned out of water blocks,

mud contamination and emulsions by the scouring and flushing action ofthe fluids.

In the second (2) area of stimulation, the action of alcohol, carbondioxide and surfactants would have significant benefit. It has beenshown that the injection of alcohol, surfactants and carbon dioxiderestores the permeability of the productive formation to gas by removingwater from the capillary pores of the formation. The surfactantdecreases the surface tension of the water causing a decrease incapillary pressure which allows the water to be more easily displaced byinjected gas. The alcohol acts as a drying agent; thus, the combinationof surfactant and dessicant forced into the formation by a gas at highpressure is very effective for removing a water block in the immediatevicinity of the well bore.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of treating a subsurface earth formation penetrated by awell bore, comprising: injecting a liquified gas into the formation andinjecting gelled alcohol containing entrained propping agents into saidformation.

2. The method of claim 1 including the step of injecting a gel breakerinto said formation.

3. The method of claim 1 wherein said gelled alcohol is gelled methanol.

4. The method of claim 1 wherein said liquified gas is carbon dioxide.

5. The method of claim 1 wherein said steps of injecting a liquified gasand said step of injecting gelled alcohol containing entrained proppingagents are performed simultaneously.

6. The method of claim 1 wherein said liquified gas and gelled alcoholcontaining the entrained propping agents are blended prior to injectioninto the formations.

7. The method of claim 1 wherein said liquified gas and gelled alcoholcontaining the entrained propping agents are mixed after each has beeninjected into the well bore.

8. The method of claim 1 wherein said gelled alcohol containing theentrained propping agents is at atmospheric pres sure prior to injectioninto the well bore.

9. The method of claim 1 wherein the entrained propping agents are sand.

10. A method of treating subsurface earth formations penetrated b aborehole, comprising: mixing ropping agents with gelled cohol, mixingsaid gelled alcoho containing the propping agents with a liquified gasand injecting the mixture into the formation surrounding said borehole.

11. The method of claim 10 including the step of adding a gel breaker tothe gelled alcohol.

12. The method of claim 10 wherein said liquified gas is liquified N 13.The method of claim 10 wherein said liquified gas is liquified CO 14. Asystem for treating subsurface earth formations comprising:

means for mixing propping agents with gelled alcohol;

means for blending the propping agents and gelled alcohol mixture with aliquified gas; and means for injecting the blend into said subsurfaceearth formations.

2. The method of claim 1 including the step of injecting a gel breakerinto said formation.
 3. The method of claim 1 wherein said gelledalcohol is gelled methanol.
 4. The method of claim 1 wherein saidliquified gas is carbon dioxide.
 5. The method of claim 1 wherein saidsteps of injecting a liquified gas and said step of injecting gelledalcohol containing entrained propping agents are performedsimultaneously.
 6. The method of claim 1 wherein said liquified gas andgelled alcohol containing the entrained propping agents are blendedprior to injection into the formations.
 7. The method of claim 1 whereinsaid liquified gas and gelled alcohol containing the entrained proppingagents are mixed after each has been injected into the well bore.
 8. Themethod of claim 1 wherein said gelled alcohol containing the entrainedpropping agents is at atmospheric pressure prior to injection into thewell bore.
 9. The method of claim 1 wherein the entrained proppingagents are sand.
 10. A method of treating subsurface earth formationspenetrated by a borehole, comprising: mixing propping agents with gelledalcohol, mixing said gelled alcohol containing the propping agents witha liquified gas and injecting the mixture into the formation surroundingsaid borehole.
 11. The method of claim 10 including the step of adding agel breaker to the gelled alcohol.
 12. The method of claim 10 whereinsaid liquified gas is liquified N2.
 13. The method of claim 10 whereinsaid liquified gas is liquified CO2.
 14. A system for treatingsubsurface earth formations comprising: means for mixing propping agentswith gelled alcohol; means for blending the propping agents and gelledalcohol mixture with a liquified gas; and means for injecting the blendinto said subsurface earth formations.